Silk is an amazing material produced naturally by various species, such as the silk moth and silk worm (Lepidoptera), bees, wasps, and ants (Hymenoptera) and spiders (arthropods). Each species' silk has its own unique set of properties.
For example, silk from the silk moth Bombyx mori is ideally suited for fashion textiles due to its light weight, soft touch, and luxurious appearance. Although silks from other species, especially spider silk, have even higher toughness and tensile strength, as well as better chemical resistance—properties that make them of great interest to industry—they have not been produced commercially to date. Spiders can produce various kinds of silk—each perfectly adapted to the specific requirements demanded by nature. Orb-web-spinning spiders produce silk fibres with mechanical properties unmatched in the natural world thereby outcompeting many synthetic fibres produced by modern technology (Slotta et al. (2012) Chemical engineering process 108, 34-49).
Spider webs can withstand high deformations for example caused by the impact of prey, due to the interplay between several specialized types of silk fibres. Dragline (or major ampullate) silk forms the frame and radii of the web and serves as a lifeline for the spider during escape. Flagelliform silk, which is more-elastic, makes up the capture spiral of the net. Other silks are responsible for reproductive purposes or as glue substance, among others (Slotta et al. (2012) Chemical engineering process 108, 34-49).
Spiders are able to produce the high-performance polymer material under environmentally friendly conditions using aqueous solutions, ambient temperature, and with low energy consumption. However, the complex mechanisms behind the seemingly simple process of natural thread formation and web construction are not yet understood and therefore cannot be readily replicated.
Many attempts have been made to mimic the spinning process at the laboratory scale and significant progress has been made, but the mechanical properties of the natural dragline fibre are still unmatched.
To produce a commercial fibre, either the natural process of silk spinning must be mimicked, or a completely new spinning process must be developed. To be commercially viable, any process must be cost-efficient and environmentally friendly.
Few data on the mechanical properties of synthetic silk fibres can be found in the literature. Most of the spinning processes create fibres that are so brittle that their mechanical properties cannot be properly measured or the resulting fibres loose performance upon drying or storage.
However, these studies do provide some useful hints about the keys to spinning silk protein. For instance, a higher-molecular-weight protein produces a more-stable fibre, as reported by Xia et al. ((2010) PNAS 107: 14059-14063). Here, recombinant proteins originating from the spider Nephila clavipes were produced and spun into a fibre displaying mechanical properties approaching those of native silk. However, such toughness was only obtained for proteins of very high molecular weight. For recombinant spider silk proteins with a molecular weight of almost 300 kD a fibre exhibiting a toughness of 141 MJ/m3 was obtained. At lower molecular weight the toughness was far inferior to native silk. These effects may be due to the reported difficulties to retain the protein at high concentrations, especially in an aqueous system.
The right combination several factors are thought to greatly improve the mechanical properties of the spun fibres. However, despite various promising approaches, the mechanical properties of natural dragline fibres have not been reproduced before.
The inventors of the present invention surprisingly found a method for producing an aqueous silk protein spinning dope solution for self-assembling polypeptides, such as spider silk polypeptides. Spinning of this dope results in fibres with a very high toughness, which tends to increase with molecular weight but is at all molecular weights superior to the toughness of silk fibres produced according to the method disclosed by Xia et. al (supra). Thus, the method of the present invention enables for the first time a unique formation of silk proteins in solution resulting in fibres with a toughness far better than reported hitherto.